fix: bulk challenge enable path (compose YAML, base images, async toggle, tooling)

- fix_dup_volumes.py: 4 canonical templates had TWO volumes: keys inside one
  service (invalid YAML -> 'mapping key volumes already defined'), which broke
  every enable for anti-alchemy/burvesigner/gemas-notes/kode-viewer.
- fjb: ghcr.io base is not anonymously pullable on this host; swapped to the
  official httpd:2.4 (its httpd.conf only uses stock modules). Added
  onlyBuiltDependencies to package.json (pnpm >=10 blocks esbuild's postinstall).
- xl + kode-viewer: node:20-slim-bookworm is not a real tag; use
  node:20-bookworm-slim. gift-voucher: buster -> bookworm.
- prebuild_images.py: build each challenge's shared services-<name> image once
  in parallel (passes a placeholder PASSWORD build-arg, since several Dockerfiles
  run chpasswd and fail on an empty arg).
- set_enabled.py / sync_all_challenges.py: batch registry flip + runtime apply
  that survives panel restarts and reports per-team results.
- Challenge toggle is now async: PATCH returns a job id, the client polls
  /api/challenges/jobs/<id> so a multi-minute build no longer blocks the panel.
  Added _SYNC_LOCK to serialize concurrent compose rewrites.

panel/prebuild_images.py

@@ -0,0 +1,88 @@
+#!/usr/bin/env python3
+"""Pre-build every challenge's shared image (`services-<name>`) in parallel.
+
+The admin toggle only reuses `image: services-<name>` when that image already
+exists locally; otherwise it keeps `build:` and each of the 4 teams rebuilds it
+separately (team1-x, team2-x, ...), which wastes minutes and gigabytes. Since a
+challenge's image content is identical for every team (the SSH password is
+applied at runtime via chpasswd, not baked into the image), one shared build
+suffices.
+
+Runs up to --jobs builds at a time and prints a per-challenge PASS/FAIL summary.
+"""
+from __future__ import annotations
+
+import argparse
+import json
+import subprocess
+import sys
+import time
+from concurrent.futures import ThreadPoolExecutor
+from pathlib import Path
+
+SERVICES = Path("/opt/gemastik18-final/services")
+REGISTRY = Path("/opt/gemastik18-final/teams/challenge_registry.json")
+
+
+def image_exists(name: str) -> bool:
+    r = subprocess.run(["docker", "image", "inspect", f"services-{name}"],
+                       capture_output=True)
+    return r.returncode == 0
+
+
+def build(name: str, force: bool) -> tuple[str, bool, str]:
+    if not force and image_exists(name):
+        return name, True, "already built"
+    d = SERVICES / name
+    if not (d / "Dockerfile").exists():
+        return name, False, "no Dockerfile (multi-service challenge)"
+    t0 = time.time()
+    # Several challenge Dockerfiles do `echo root:${PASSWORD} | chpasswd`, so an
+    # empty build-arg makes the RUN step fail. Pass a throwaway value: the real
+    # per-team password is applied at container start via chpasswd, never baked
+    # into the image, so this value never matters.
+    r = subprocess.run(
+        ["docker", "build", "-t", f"services-{name}",
+         "--build-arg", "PASSWORD=prebuild-placeholder", "."],
+        cwd=d, capture_output=True, text=True,
+    )
+    if r.returncode == 0:
+        return name, True, f"built in {time.time()-t0:.0f}s"
+    tail = [l for l in r.stderr.splitlines() if l.strip()][-1:] or ["unknown error"]
+    return name, False, tail[0][:160]
+
+
+def main() -> int:
+    ap = argparse.ArgumentParser()
+    ap.add_argument("names", nargs="*", help="challenge names (default: all in registry)")
+    ap.add_argument("--jobs", type=int, default=4)
+    ap.add_argument("--force", action="store_true", help="rebuild even if present")
+    args = ap.parse_args()
+
+    if args.names:
+        names = args.names
+    else:
+        reg = json.loads(REGISTRY.read_text())
+        names = [c["name"] for c in reg["challenges"]]
+
+    print(f"building {len(names)} challenge images with {args.jobs} parallel jobs\n", flush=True)
+    t0 = time.time()
+    results: list[tuple[str, bool, str]] = []
+    with ThreadPoolExecutor(max_workers=args.jobs) as ex:
+        for res in ex.map(lambda n: build(n, args.force), names):
+            results.append(res)
+            mark = "PASS" if res[1] else "FAIL"
+            print(f"[{mark}] {res[0]:16s} {res[2]}", flush=True)
+
+    ok = [r for r in results if r[1]]
+    bad = [r for r in results if not r[1]]
+    print(f"\n{len(ok)}/{len(results)} images ready in {(time.time()-t0)/60:.1f} min")
+    if bad:
+        print("\nfailures:")
+        for n, _, why in bad:
+            print(f"  {n}: {why}")
+    return 1 if bad else 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())